1. Field of The Invention
The present invention relates generally to display devices, and in particular, to an electrochromic pane arrangement.
2. Description of the Related Art
Electrochromic pane arrangements for use as display devices are known in the art. Such electrochromic panes may typically have a first transparent electrode substrate or glass pane, a second electrode substrate or glass pane arranged parallel to and at a distance from the first glass pane, and an ion conducting layer joining the two electrode substrates. The ion layers may consist of a polymer matrix containing at least one salt including difficult to oxidize anions, and at least one dopant suitable for the provision of cations, preferably metal ions. A surface of the first electrode substrate faces toward the ion conducting layer and bears a first transparent electrode layer. Adjacent to it is an actively electrochromic layer in surface contact with the ion conducting layer. The electrochromic layer is preferably of tungsten trioxide, having a reversibly variable transparency and/or color resulting from absorption or emission of electrons via the first electrode layer and acceptance or emission of ions of the dopant from or to the ion conducting layer. A surface of the second electrode substrate faces towards the ion conducting layer and bears a second transparent electrode layer. Adjacent to it is a transparent layer in surface contact with the ion conducting layer. The transparent layer is suitable for the reversible storage of charge, preferably by ion insertion.
An electrochromic laminated pane of this type is known from DE-A 41 16 059, where the two individual panes are joined to one another by means of a polyvinylbutyral layer which represents the ion conducting layer. The polyvinylbutyral layer, which is a film of polyvinylbutyral sufficiently well-known for the manufacture of laminated safety glass panes, contains LiClO.sub.4 as dopant.
Electrochromic pane arrangements do not as a rule evidence any electrochromic properties in their freshly manufactured state, unless the electrochromic layer and/or the charge storage layer have been precharged by the insertion of positive ions. Otherwise, only after so-called preconditioning, does the pane arrangement acquire the property of variable light transmission. For the purpose of preconditioning, the pane arrangement is switched several times at low voltages and low cycle time after connection of a DC voltage. One cycle always consists of a colouration and bleaching period. During colouration and bleaching, identical or differing voltages can be applied, but with reversed polarities, where the pane incorporating the actively electrochromic layer, normally taking the form of a WO.sub.3 layer, is connected during colouration as a cathode, but on the other hand as an anode during bleaching.
A first type of preconditioning is characterized by successive increase of the switching times and of the voltages applied.
It has been found favourable at the beginning of preconditioning to choose the colouration period of longer duration than the bleaching period, as then lower transmission values can be induced in the pane arrangement in stages. This method of preconditioning leads to a high transmission range, but is very time-consuming on account of the high number of cycles, this time requirement naturally increasing the cost of manufacture of the pane arrangement.
If it is desired to avoid the considerable time involved in the type of preconditioning used in the procedure described above, it is also known in the state of the art that the individual electrode layers can be charged with lithium ions. Either the actively electrochromic layer or the above-mentioned ion storage layer is charged with for example lithium ions before assembly to form the electrochromic pane arrangement. This is done by immersing the individual pane to be preconditioned in each case in a liquid electrolyte solution and applying a DC voltage. As counter electrode, there is a metal plate or also a glass pane matched to the pane size which is provided with an electronically conductive layer, where the electrolyte solution is composed as a rule of propylene carbonate and a lithium salt. By these means, it is possible to achieve a considerable reduction in the preconditioning time in comparison with the type of preconditioning described above, but there is the disadvantage that an additional operation is necessary, with the corresponding cleaning problems and difficulties involved in the production of the glass sandwich from the preconditioned individual panes, so that the production of electrochromic pane arrangements is relatively time-consuming, even with this second type of preconditioning.
An electrochromic pane arrangement of the type mentioned at the beginning is known from EP-B 0 083 988, where the ion conducting layer takes the form of a cast resin layer, where propylene carbonate is used as solvent for the conducting salt LiClO.sub.4. Here as well, the preconditioning problems described above occur.
In EP-B 0 098 416, an electrochromic pane arrangement similar to that of the generic type is described, where for the ion conducting layer a polymethyl methacrylate-based or a copolymer of methyl methacrylate/methacrylic acid-based lithium ion conductor is used. The preconditioning problems of the type described further above are also present with the procedure according to this publication.
Finally, an electrochromic pane arrangement of another type is known from EP-A 0 495 220, where no electrochromic layer is used, but a metal, for example Cu and/or Bi, is deposited on a glass surface, as a result of which darkening occurs. This reaction is reversible. On the counter-electrode not defined in detail, the anion of the salt in question is oxidized, this action also being reversible. To increase the solubility of CuCl and/or BiCl.sub.3, a further salt can be added in the liquid electrolyte whose anions can form complexes with Cu. Lithium bromide is used in preference for this purpose. Finally, a salt with non-complexing anions can also be used, LiClO.sub.4 being used in preference. According to the data of EP-B 0 495 220, the switching properties should be improved thereby. A problem with this procedure is the fact that in the event of a fracture of the pane arrangement or any other form of leak, the electrolyte liquid can escape, which will occasion considerable contamination problems. In addition, the use of the liquid electrolyte, in which lithium bromide and where applicable LiClO.sub.4 are present for an increase in solubility or an improvement of the switching properties, does not permit the design of large-area switching elements. Finally, only relatively low operating voltages of not more than 2 volts are possible, as otherwise decomposition reactions can occur in the liquid electrolyte.